Objectives: 1) We will continue studies of the extent of X chromosomal inactivation. 2) We will continue attempts to reactivate the inactive X as a means of understanding regulatory mechanisms in mammalian cells. 3) We will continue to study contact-feeding in relation to improving our ability to isolate novel X-linked or autosomal mutations in normal human cells. 4) We will continue to study hybrids derived exclusively from strains of human diploid cells as a means of detecting genetical heterogeneity among inborn errors of metabolism. 5) We will continue to select for euploid epithelial cells and to use these cells in culture to study the control of differentiated functions.